This invention relates generally to local-area computer networks, and more particularly, to an interface unit for connection between a communication bus and any of a variety of devices, such as computers, display and keyboard terminals, and printers. As computers have become less expensive and more numerous, there has been an increasing demand for simple techniques that permit interconnection of computers, terminals and computer peripherals, such as printers. When connected to a network, relatively inexpensive desk-top computers can gain access to larger computers and to high-speed printers and large-capacity memory devices.
Another advantage of computer networks is that the users of the interconnected devices may communicate with each other by what is now referred to as electronic mail. In a typical electronic mail system a "host" computer accepts and temporarily stores messages transmitted from one user to another. The messages may be immediately forwarded to their destinations, or may be held until the intended recipient connects with the host computer and "picks up" his electronic mail. In any event, a host computer is required to act as the "post office" in most mail systems of this type.
Of course, computers may be interconnected through telephone lines, using a relatively simple modulator-demodulator (modem) for connecting each computer or terminal to the telephone circuit. Again, at least one device in such a network, usually the host computer, has to supply the "intelligence" for directing the steps of communication among the connected devices. This invention, however, is concerned with a different type of network, referred to as a local-area network, in which the connected devices are usually located in a relatively small geographical area. For example, many offices of a corporation may have a number of terminals, computers and other devices connected to a single local-area network. This would allow all of the network users to share the same data bases, to have access to the same computer programs, and to communicate with each other in day to day activities. More specifically, users could access the same word-processing programs and text files, could use a common telephone directory, and could generate and distribute internal memoranda and reports over the network.
A variety of local-area networks have been developed in recent years, employing differing network topologies and differing communication protocols. This invention is specifically concerned with networks having a simple bus topology, to which each of the connectable devices, or "nodes," is connected through a bus interface unit. Access to a communication bus may be regulated by a number of different approaches. One is to allocate time to each connected node on some uniform time-division multiplexing basis. A more common approach is to allow any node to transmit on the bus when ready to do so, but to provide some means for sensing whether another transmission is also taking place. The latter is sometimes referred to as a carrier-sense multiple access system. The present invention is embodied in a system employing this latter approach, although the principles of the invention could be applied to systems employing other schemes for controlling bus access.
The use of a bus interface unit between a network device and the communication bus is not new in itself. U.S. Pat. No. 4,161,786 to George T. Hopkins et al. discloses a digital bus communication system in which bus interface units are connected between the bus and the "subscriber devices." The bus interface units in the Hopkins system appear to be used in part for participating in the control of access to the bus.
Broadly stated, the function of any bus interface unit in a local-area network is to provide modulation and demodulation of messages, and to some degree, to format messages for transmission and to detect and decode received messages. Although a variety of local-area network equipment is available today, the establishment of a communication link from a terminal to a computer on the network is not conveniently made. Moreover, a certain amount of computing power in the nature of system overhead, either at the host computer or at an intelligent terminal, is required to establish and maintain the link. Communication between relatively "dumb" terminals is usually made through a host computer, and then not in a conveniently conversational mode.
Ideally, a bus interface unit for use in a local-area network should render the bus as "transparent" as possible to the user. In other words, each user should be able to communicate with any computer, device, or other user terminal on the network without concern for the nature of the bus, the message formats used, or the physical location or address of the intended destination. Furthermore, a bus interface unit should ideally be small in size and relatively inexpensive, unlike most of the units that have been available for similar purposes. The present invention meets or exceeds these ideal requirements.